correct documentation link

[Contents:]

This package provides a modified version of

"fix npt"_fix_nh.html which directly controls the Cauchy (true) stress

instead of the second Piola-Kirchhoff stress, as presented in:

This package provides a modified version of "fix npt"_fix_nh.html

which directly controls the Cauchy (true) stress instead of the

second Piola-Kirchhoff stress, as presented in:

R. E. Miller, E. B. Tadmor, J. S. Gibson, N. Bernstein, and F. Pavia,

“Molecular Dynamics at Constant Cauchy Stress,”  J. Chem. Phys. 144, 184107 (2016).

[Supporting info:]

Example input is in the examples/USER/cauchy folder.  Usage instructions are part of

the "fix npt"_fix_nh.html description.

Example input is in the examples/USER/cauchy folder.  Usage instructions are

in the "fix npt/cauchy"_fix_cauchy.html documentation.

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